Auto merge of #25494 - pshaughn:leadingzeroes, r=jdm

Make sure input value=12:30:01 doesn't turn into 12:30:1

Fixed convert_valid_normalized_local_date_and_time_string to add the mandatory leading 0 before seconds values that needed it, added tests to see that various-length strings with zeroes in them would roundtrip properly.

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- [X] These changes fix #25493

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components/script/dom/bindings/str.rs

@@ -474,7 +474,15 @@ impl DOMString {
                 "{:04}-{:02}-{:02}T{:02}:{:02}",
                 year, month, day, hour, minute
             );
+        } else if second < 10.0 {
+            // we need exactly one leading zero on the seconds,
+            // whatever their total string length might be
+            self.0 = format!(
+                "{:04}-{:02}-{:02}T{:02}:{:02}:0{}",
+                year, month, day, hour, minute, second
+            );
         } else {
+            // we need no leading zeroes on the seconds
             self.0 = format!(
                 "{:04}-{:02}-{:02}T{:02}:{:02}:{}",
                 year, month, day, hour, minute, second